High-Temperature Creep of Polycrystalline Magnesia: I,Effect of Simultaneous Grain Growth *

The creep of pure magnesia (99.9 +% MgO) was tested in transverse bending at temperatures from 1200° to 1500°C, strain rates near 10⁻²%/hr, and grain sizes of 4 to 50μ. In most cases, grain growth during the test affected the apparent creep behavior more than all the other variables combined. An analytical graphical method was used to separate the grain growth effect from other effects and to obtain more meaningful creep data. Creep occurred primarily by a viscous mechanism (Nabarro-Herring type, cation-lattice-diffusion controlling) with a minor amount of plastic creep (dislocation climb). The agreement with previous creep data was good.     

High-Temperature Failure of Polycrystalline Alumina: II, Creep Crack Growth and Blunting

Creep crack growth in fine-grain alumina is measured by using surface cracks. A narrow power-law crack growth regime occurs at both 1300° and 1400°C, wherein the power-law exponent and activation energy are comparable to steady-state creep values. Asymptotic crack velocity behavior is exhibited near both the critical stress intensity factor, K_C , and the crack growth threshold, K_th . The threshold occurs near 0.4 K_1C at both 1300° and 1400°C and is associated with a transition in the size and distribution of damage. Displacement measurements indicate that crack tip damage exerts a strong influence on the displacement field, as predicted by recent theories. Furthermore, use of the stress intensity factor as a loading parameter does not produce adequate correlation with displacement measurements and is, therefore, not strictly suitable for nonlinear creeping ceramic poly crystals.     

High-Temperature Compressive Creep of Polycrystalline Mullite

Aluminosilicates of three compositions with mullite as the major phase were synthesized by a sol-gel process and characterized with bulk and microchemical analyses and microstructural observation. An apparatus for measuring the compressive creep up to 1900 K with a sensitivity of ±1 μm was constructed and used to measure the creep of singlephase mullite, mullite with second-phase glass, and mullite with second-phase corundum. Measurements in air at stresses of 15 to 100 MPa and temperatures of 1471 to 1724 K determined that samples with second-phase glass crept more rapidly than single-phase mullite or mullite with secondphase corundum. The apparent creep activation energies determined at 100 MPa were 742 kJ/mol for the mullite containing glass, 819 kJ/mol for the single-phase mullite, and 769 kJ/mol for the mullite with second-phase corundum. The stress exponents determined at 1724 K were 1.6 for the mullite plus glass, 1.5 for the single-phase mullite, and 1.2 for the mullite with α-Al₂O₃. The creep behavior of the aluminosilicates containing glass were consistent with rate control by the viscous flow of the glass and the measured creep rates were in good agreement with creep rates calculated from a model by Dryden. The creep behavior of the completely crystalline aluminosilicates was consistent with rate control by diffusional creep.     

High-Temperature Creep and Cavitation of Polycrystalline Aluminum Nitride

Dense, polycrystalline AlN samples of grain size between 1.8 and 19 μm were fabricated by hot-pressing. Bar-shaped samples were subjected to creep in four-point bending under static loads in nitrogen atmosphere. The outer fiber stress was varied between 20 and 140 MPa and the temperature between 1650 and 1940 K. Steady-state creep rate, dɛ/d t was proportional to σ ⁿ d ^{− m} where the stress exponent, n , was between 1.27 and 1.43 and grain-size exponent, m , between ∼ 2.2 and ∼ 2.4. The activation energy for creep ranged between 529 and 625 kJ/mol. Both round (r type) and wedgeshaped (w type) cavities were observed in electron micrographs of the deformed samples. No noticeable change in the dislocation density was observed. Contribution of cavitation to the creep rate was estimated using an unconstrained cavity model. Based on this study it is concluded that the dominant mechanism of creep in polycrystalline AlN is diffusional.     

High-Temperature Creep and Dislocation Etch Pits in Polycrystalline Beryllium Oxide

Compressive creep of high-density polycrystalline beryllium oxide was investigated in the range 1850° to 2050°C. Creep rate was dependent on the applied stress to the 2.5 power, and the apparent activation energy for creep was 145 kcal/mole. Etch pit studies showed that the dislocation density in tested specimens was two orders of magnitude greater than that in assintered material. The diffusion process controlling creep was ascribed to volume diffusion of the anion. The deformation behavior was governed by dislocation motion.     

High-Temperature Creep Behavior of Polycrystalline SrZrO3

The high-temperature creep behavior of sintered polycrystalline SrZrO₃ containing 1.35 wt% Fe₂O₃ was investigated as a function of temperature, stress, grain size, and strain level over the ranges 1160° to 1275°C, 780 to 3110 psi, 0.45 to 2.04 μm, and 0.0014 to 0.014, respectively. A constant-load 4-point (pure bending) method was used to load the specimens. The creep rate is time-dependent, decreasing exponentially with strain, i.e.     , where the decay constant (β=118, measured at the 1560 psi stress level over the strain range 0.0014 to 0.014) is independent of temperature and grain size. No significant grain growth occurred during creep. The activation energy of 169±10 kcal/mol obtained for creep is relatively independent of temperature, stress, grain size, and strain level over the ranges investigated. The creep rate is directly proportional to the cube of the stress and the reciprocal of the grain size; this result is consistent with nonviscous creep theories based on dislocation generation and climb as the rate-controlling deformation mechanism.     

Creep of Dense, Polycrystalline Magnesium Oxide

Creep of polycrystalline MgO was studied using four-point transverse bending at 1380° to 1800°K and stresses from 1000 to 5000 psi. The effects of temperature, stress, and grain size on the creep rate were determined for grain sizes from 2 to 20μ. Activation energies for creep decreased sharply with increasing grain size from 96,000 cal/mole at 2μ to 54,100 cal/mole at 5.5μ and then remained constant over the grain-size range 5.5 to 20μ. Creep was attributed in part to a stress-directed diffusional mechanism controlled by extrinsic oxygen ion diffusion in the 5.5 to 20μ grain sizes, although the calculated ionic self-diffusion rates were higher than those predicted by the Nabarro-Herring theory. It is suggested that the discrepancy may be due to a vacancy formation mechanism, which is consistent with the observed formation of dislocation substructure and preferentially distributed porosity during creep, as well as with the observed decrease in creep rate with increasing creep strain.     

High-Temperature Failure of Polycrystalline Alumina: I, Crack Nucleation

A combination of techniques has been used to compare the compositional and microstructural characteristics of two commercially hot-pressed aluminas. Second-phase material containing Ni was observed in one material, mainly as small intergranular particles. Thermal treatment in air caused Ni to concentrate at the surfaces, in small precipitates. Large-grained heterogeneities, with Ti at the core, were identified in the same material. Hot-pressing flaws were observed in the second material. The large-scale heterogeneities act as crack nucleation sites during creep. Stress concentrations associated with these heterogeneities are considered to contribute to the premature crack nucleation, in conjunction with the presence of localized regions of amorphous second phase.     

High-Temperature Failure of Polycrystalline Alumina: III, Failure Times

High-temperature failure data have been obtained for two polycrystalline aluminas, revealing a strongly stress-dependent failure time. The observed stress dependence has been compared with the dependence predicted by models of the nucleation, growth, and coalescence stages of failure. Only models of the coalescence phase indicate sufficient nonlinearity to be a plausible explanation of the data. Further observations and measurements of coalescence, involving continuous crack nucleation and shear-band formation, are identified as requirements for further understanding of the rupture process.     

Kinetics and Mechanisms of High-Temperature Creep in Silicon Carbide: II, Chemically Vapor Deposited

Chemically vapor deposited (CVD) silicon carbide was subjected to constant compressive stresses (110 to 220 MN/m²) at high temperatures (1848 to 2023 K) in order to determine the controlling steady-state creep mechanisms under these conditions. An extensive TEM study was also conducted to facilitate this determination. The strong preferred crystallographic orientation of this material causes the creep rate to be very dependent on specimen orientation. The stress exponent, n , in the equation εασⁿ was calculated to be 2.3 below 1923 K and 3.7 at 1923 K. The activation energy for steady-state creep was determined to be 175 ± 5 kJ/mol throughout the temperature range employed. At temperatures between 1673 and 1873 K, the controlling creep mechanism for CVD Sic is dislocation glide, which is believed to be controlled by the Peierls stress. Although the activation energy does not change, the increase in the stress exponent for samples deformed at 1923 K suggests that the controlling creep mechanism becomes dislocation glide/climb controlled by climb.     

Creep of Polycrystalline Thorium Dioxide

The creep rate of polycrystalline thorium dioxide is reported for 1400° to 1800°C and for 4000, 7500, and 11,000 psi. The microstructure of deformed specimens contained intergranular voids which formed by the apparent growth and coalescence of pores along the grain boundaries. Grain-boundary sliding also occurred during creep. The strain rate-stress relation indicates a viscous type of creep process. A mechanism of creep is proposed which involves a diffusional transport of material and which accounts for the appearance of the intergranular voids.     

High-Temperature Fatigue Behavior of Polycrystalline Alumina

The strength and fatigue behavior of a 99.5% polycrystalline alumina were measured as a function of temperature. Both the strength and fatigue behavior remained essentially constant up to 500°C; from 800° to 1100°C the strength and fatigue resistance decreased markedly and at >1100°C macroscopic creep was observed. It is believed that the decrease in strength and fatigue resistance is caused by a grain-boundary glassy phase enhancing subcritical crack growth. Proof-testing at room temperature was effective in improving the strength distributions at both room temperature and 1000°C; however, at 1000°C it was not effective, due to crack growth during the proof test. The good agreement between proof-test results and fracture-mechanics theory indicates that the same flaws control the strength at room temperature and at high temperatures.     

Compressive Creep of Polycrystalline Beryllium Oxide

Compressive creep of polycrystalline beryllium oxide was studied in the temperature range 2500° to 2800°F. The apparent activation energy for creep was 96.0 kcal per mole and the creep rate was linearly dependent on the applied stress. Results were consistent with the Nabarro-Herring creep mechanism. Experimental evidence showed that extrapolation of data to 3200°F was possible.     

Solubility of Magnesia in Polycrystalline Alumina at High Temperatures

High-purity Al₂O₃ compacts were doped with 0–350 ppm (by weight) of MgO using a liquid immersion technique and equilibrated at temperatures between 1700° and 2000°C under hydrogen. The solubility limits of MgO in Al₂O₃ at temperatures of 1720° and 1880°C were very low, ∼75 and 175 ppm, respectively. Variation of MgO solubility with temperature could be represented by the equation, ln Mg/Al = 3.80–2.63 × 10⁴/ T . The small MgO solubilities were understood by the high enthalpy (326 kJ/mol) of solution. The results of this study suggested that previous investigations on sintering and grain-growth mechanisms in MgO-doped Al₂O₃ were probably not done in single-phase Al₂O₃ solid solutions. However, the conclusions on sintering and grain-growth mechanisms in prior research work in MgO-doped A₂O₃ may be correct. The effects of SiO₂ impurity and grain size on MgO solubility are discussed. Previous grain-growth experiments in MgO-doped Al₂O₃ are described that demonstrate the clearest evidence for grain-boundary mobility controlled by a solid-solution mechanism.     

High-Temperature Plastic Deformation of Polycrystalline Beryllium Oxide

High-density specimens were plastically deformed under four-point transverse bending. Tests were conducted in vacuum in the region 1400° to 1700°C under stresses of 1000 to 4500 psi. The activation energy for creep was 99.0 kcal/mole. Creep rate was directly proportional to the applied stress and inversely proportional to the square of the grain diameter. The deformation behavior is ascribed to a Nabarro-Herring type mechanism. Results show that creep was the same in tension and compression.